Zn precursors for Cu2ZnSnS4 thin films

Author’s Accepted Manuscript Impact of pre-alloying of sputtered Cu/Sn/Zn precursors for Cu2ZnSnS4 thin films Sheng-Wen Fu, Hui-Ju Chen, Shih-Hsiung Wu, Hsuan-Ta Wu, Chuan-Feng Shih www.elsevier.com

PII: DOI: Reference:

S0167-577X(16)30206-3 http://dx.doi.org/10.1016/j.matlet.2016.02.046 MLBLUE20325

To appear in: Materials Letters Received date: 10 January 2016 Revised date: 9 February 2016 Accepted date: 11 February 2016 Cite this article as: Sheng-Wen Fu, Hui-Ju Chen, Shih-Hsiung Wu, Hsuan-Ta Wu and Chuan-Feng Shih, Impact of pre-alloying of sputtered Cu/Sn/Zn precursors for Cu2ZnSnS4 thin films, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2016.02.046 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Impact of pre-alloying of sputtered Cu/Sn/Zn precursors for Cu2ZnSnS4 thin films Sheng-Wen Fu1, Hui-Ju Chen1, Shih-Hsiung Wu1, Hsuan-Ta Wu1 and Chuan-Feng Shih12* 1

Department of Electrical Engineering, National Cheng Kung University, Tainan, Taiwan

2

Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan, 70101, Taiwan *E-mail address: [email protected]

The Cu2ZnSnS4 (CZTS) films were prepared by sulfurizing the sputtered Cu-Sn-Zn precursors. The precursors were fabricated by sequentially sputtering of Cu, Sn and Zn metallic layers onto Mo-coated glass substrate. The effects of alloying before sulfurization on the structural and optical characteristics of the CZTS films were presented. Two independent sulfurization processes were carried out and compared. The sulfurization of the reference was completed in the tube furnace at 550 °C for 2 h. In the two-stage heat treatment approach, the precursor was alloyed at various temperatures before the sulfurization. Experimental results showed that alloying at 170 °C was crucial to improve the CZTS films, yielding uniform and dense grains with single kesterite CZTS phase. Keywords Solar energy materials; Thin films 1. Introduction Kesterite CZTS is a p-type semiconductor with an optical band gap and a high absorption coefficient. The CZTS films have been fabricated by physical and chemical techniques including sputtering, thermal evaporation, electron beam evaporation, sol-gel deposition and electrodeposition. However, the conversion efficiency of CZTS based solar cells has been still

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too low to commercialization, mainly because of the difficulties in control of the secondary or ternary phases within the CZTS. The occurrence of the undesired phases complicated the compositional control of the CZTS [1]. The presence of some second phases within the CZTS films were also recombination centers for carriers that degraded the PVs performance [2]. Matsumura et al. reported that the pre-heating treatment before the high-temperature sulfurization had a significant effect on the quality of the electrodeposited CZTS films [3]. Tanaka et al. pre-annealed the sol-gel prepared CZTS precursors in air before the sulfurization to diminish the number of voids in the final CZTS films [4]. However, the study investigated on the alloying effect of the sputtered metal precursor is still absent. The paper reports a two-step heat treatment process to fabricate the CZTS films that alloying the sputtered CZT precursor followed by the sulfurization. The effects of alloying the precursor before sulfurization on the structural and optical characteristics of the CZTS films were presented. Experimental results showed that the alloying at 170 °C was crucial to improve the CZTS films with uniform and densely grains. 2. Experimental details The metallic CZT precursor was fabricated by sequential sputtering of Cu (DC power, 100 W), Sn (RF power, 80 W) and Zn (RF power, 80 W) layers onto Mo-coated substrate. Two different sulfurization procedures were performed. Firstly, the reference sample was put in the graphite container with 2g sulfur powder. The CZTS crystallized in a tubular furnace filled with N2 gas at 1atm. Samples were heated up to 550 °C with a heating rate of 10 °C min-1 and kept for 2 hours, and then cooled down naturally to room temperature with a rate of ~ 4 °C min-1. Secondly, the two-step sulfurization (TSS) approach alloyed the CZT precursors before the

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sulfurization. The samples were alloyed at 170 °C, 220 °C and 250 °C for 1 h in vacuum, labeled as CZTS-170, CZTS-220 and CZTS-250, respectively. The samples were cooled down after the alloying treatment, and then ramping up to 550 °C for sulfurization. The sulfurization process of the TSS samples was the same with the reference. The morphology of the CZTS films was characterized by field-emission scanning electron microscopy (FE-SEM, HITACHI SU8000, Japan). The composition was determined by EDS attached to the FE-SEM. The phase crystallinity of the annealed CZTS films was investigated by high resolution x-ray diffraction (HR-XRD, BRUKER D8 SSS, Germany). Micro-Raman and PL measurements (Horiba Jobin Yvon Labram HR, Japan) were performed at room temperature with 532 nm-laser. The optical analysis was performed using UV-vis-NIR spectrophotometer (HITACHI U4100, Japan) equipped with an integrated sphere. 3. Results and Disscusion Figure 1 (a) shows the FE-SEM micrographs of the reference was rough with aggregated fine grains that formed large clusters. Figures 1 (b)-(d) show the SEM surface morphology of the CZTS-170, CZTS-220 and CZTS-250, respectively. The surface of CZTS-170 was smooth and uniform (Fig. 1 (b)). When the alloying temperature was higher than 220 °C, the CZTS films became rough and some voids formed, demonstrating that the high alloying temperature contributed to the increase in the mismatch between the metal alloys and the substrate. Moreover, some cracks were found in CZTS-250 (Fig. 1 (d)). These results were different with the previous reports, in which a high-temperature pre-heating was adopted that was unsuitable for the sputtered CZTS films [3-4]. In our case, voids and uneven surface morphology were reduced when CZTS films was fabricated by pre-alloying of the CZT

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precursors at 170 °C. When the alloying temperature was higher than 220 °C, the CZTS films became rough and some voids formed. Figures 2 (a)-(b) show the interfacial of the reference exhibited serious voids and pores near the CZTS/Mo interface. The formation of cracks was attributed to the Kirkendall inter-diffusion of the metal precursor during sulfurization [5]. High diffusivity of Cu drove it to migrate toward the film surface, forming Cu-S compounds and Cu+ vacancy (VCu). The inter-diffusion of Cu vacancies formed the voids in the resulting films. Unfortunately, the occurrence of pores at the CZTS/Mo interface degraded the solar cell properties by limiting the carrier transport and collection. Differently, uniform, dense and homogeneous CZTS grains with very little pores were observed in the CZTS-170 samples (Figs. 2(c)-(d)). The CZTS grain size was large and the adhesion with the Mo substrate was good that meet the requirement of the high-efficiency solar cells. The cracks were possibly originated from mismatch and low bonding strength between CZTS and Mo substrate that were significantly eliminated through the alloying process. The stoichiometry of the CZTS films was critically for the solar cells performance. Composition of CZTS was measured by EDX and shown in Table 1. Typically, the Cu-poor and Zn-rich CZTS absorbers were required for CZTS devices that avoided the formation of the unfavorable CuZn antisite [6]. The reference was Cu-rich, but the (CZTS-170) was Cu-poor ([Cu]/([Zn]+[Sn])=0.83) and Zn-rich ([Zn]/[Sn]=1.22). Low metal sulfur ratio was obtained in the referenced sample owing to the formation of Cu-S compounds in the CZTS surface, or the composition inhomogeneity. For the CZTS-220 and CZTS-250 samples, the zinc-loss problem was severe, owing to the high vapor pressure of Zn.

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The alloying process prevented the hazard Zn-loss process that in turns improved the crystallization of the CZTS films during the sulfurization [7]. Figures 3 (a)-(c) show XRD patterns of the reference and CZTS-170 samples. The major diffraction peaks of CZTS-170 corresponded to the kesterite CZTS structure (JCPDS No. 01-075-4122) (Figs. 3 (a)). Obviously, the secondary phases such as Cu2−xS (JCPDS No. 00-020-0365) and CuS (JCPDS No. 03-065-3928) were detected in the reference, shown in Figs. 3 (b)-(c). The occurrence of the undesired phases was ascribed to the uncompleted sulfurization. The as-deposited CZT precursor was almost amorphous, which has strong tendency to react with the sulfur vapor during the sulfurization and resulting in the sulfur-related compounds. No second phases were observed in CZTS-170. Therefore, we concluded that the alloying of the CZTS at a low temperature before sulfurization increased the crystallization and eliminated the second phase of the films. To further examine the existence of the second phases, Raman spectroscopy was adopted. Figures 3 (d)-(e) show Raman analysis of the reference and CZTS-170. The dominant Raman peak at 337-338 cm-1 and other peaks at 288 cm-1, 354 cm-1 and 372 cm-1 were associated with the kesterite CZTS phase. Notably, Fig. 3 (d) shows a Raman peak at 351cm-1, indicating the ZnS phase was embedded in the reference. Figure 3 (e) shows the Raman shifts of the CZTS-170. No signals from ZnS and CTS phases (267cm-1, 303 cm-1, 356 cm-1) were detected, confirming that the single kesterite CZTS was obtained in the CZTS-170 sample. The Raman spectroscopy shows consistent observation with the XRD analysis, indicating that the alloying process improved the phase purity of the CZTS films. UV-vis-NIR optical absorption spectra have been recorded at room temperature. Thereafter, the absorption coefficient α corresponding to the photon

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energy hυ was used to evaluate the optical band gap [8], ( the optical constant, gap energy, and

)

(

) , where A is

is Planck’s constant, υ is the frequency of light, Eg is the optical band ⁄ for direct-band gap semiconductors. The plot of (

incident photon energy (

) versus the

) was fitted and shown in Fig. 3 (f). The optical band gap was

determined to be 1.35 eV and 1.40 eV for the reference and CZTS-170, respectively. A possible reason for the blue shift in the band gaps was attributed to the chemical composition variation in the CZTS films. Babu G. et al. [9] investigated that the band gap of CZTSe shifted to high energy when the Cu/(Zn+Sn) decreased, due to the change in the degree of p-d hybridization between Se p-levels and Cu d-levels. This fact was similar with the hybridization in S p-levels and Cu d-levels in CZTS [10]. The blue shift in the band gap was possibly ascribed to the Moss-Burstein shift [11]. Typically, the Cu vacancies were easily formed in the Cu-poor and Zn-rich condition, therefore, the blue shift in the band gap caused by the excess holes reduced the valence band maximum. The PL analysis of the CZTS films exhibited a blue shift as shown in the inset of Fig. 3 (f). 4. Conclusion The morphology of the CZTS films was strongly dependent on the alloying temperature of the sputtered CZT precursors. Voids were reduced and surface became smooth when the CZTS films were fabricated by alloying the CZT precursors at 170 °C. When the alloying temperature was higher than 220 °C, the CZTS films became rough and some voids formed. The TSS process improved the morphology quality of CZTS, yielding uniform, compact and dense grains with few pores (CZTS-170). HR-XRD and Raman analysis demonstrated TSS was favorable to achieve the single-phase CZTS without secondary phases or ternary phases,

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whereas the reference exhibited undesired phases such as Cu2-xS, CuS and ZnS. The occurrence of the secondary phases was ascribed to the inter-diffusion of Cu and uncompleted sulfurization during the one-step sulfurization. References [1] Dhakal TP, Peng CY, Reid Tobias R, Dasharathy R, Westgate CR. Solar Energy. 2014;100:23-30. [2] Muhunthan N, Singh OP, Singh S, Singh VN. Int J Potoenergy. 2013;2013:1-7. [3] Jiang F, Ikeda S, Harada T, Matsumura M. Adv Energy Mater. 2014;4: 1301381. [4] Lin Y, Ikeda S, Septina W, Kawasaki Y, Harada T, Matsumura M. Sol Energ Mat Sol Cells. 2014;120:218-25. [5] Liu X, Mayer MT, Wang D. Angew Chem Int Ed Engl. 2010;49:3165-8. [6] Maeda T, Nakamura S, Wada T. Jpn J Appl Phys. 2011;50:04DP7. [7] Platzer-Björkman C, Scragg J, Flammersberger H, Kubart T, Edoff M. Sol Energ Mat Sol Cells. 2012;98:110-7. [8] Fox AM. Optical Properties of Solids: Oxford University Press; 2001. [9] Suresh Babu G, Kishore Kumar YB, Uday Bhaskar P, Raja Vanjari S. Sol Energ Mat Sol Cells. 2010;94:221-6. [10] Bao W, Ichimura M. Int J Potoenergy. 2015;2015:1-6. [11] Tanaka K, Fukui Y, Moritake N, Uchiki H. Sol Energ Mat Sol Cells. 2011;95:838-42.

Figure captions

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Fig. 1 FE-SEM top view of annealed CZTS films for (a) reference, (b) CZTS-170, (c) CZTS-220 and (d) CZTS-250. Fig. 2 FE-SEM cross-sectional view of annealed CZTS films for (a-b) reference and (c-d) CZTS-170. Fig. 3 (a)-(c) HR-XRD patterns of annealed CZTS films. Raman spectra of CZTS films for (d) reference (e) CZTS-170. (f) Optical band gap determination of CZTS films (Inset: PL analysis of the CZTS films).

Highlights

1. The alloying process improved the phase purity of CZTS thin films.

2. The cracks near the CZTS/Mo interface were eliminated.

3. High-temperature pre-heating was unsuitable for sputtered CZTS films.

4. The alloying treatment was a potential way to obtain high-efficiency CZTS solar cells.

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Table captions Table 1 Compositional analysis of annealed CZTS films.

Sample

Alloying treatment

CZTS thin films (after sulfurization) [Cu]/([Zn]+[Sn]) [Zn]/[Sn] Metal/S

Reference

No Alloying

1.01

0.93

0.84

CZTS-170

170 °C-1 h

0.83

1.22

1.09

CZTS-220

220 °C-1 h

1.07

0.88

1.14

CZTS-250

250 °C-1 h

1.12

0.84

1.09

Fig. 1

9

Fig. 2

2 theda (degree) 25

30

35

40

45

50

55

60 (b)

Reference CZTS-170

Intensity (a.u.)

Reference CZTS-170

Cu2-xS

CuS

25

26

27

Cu2-xS

28

29

CuS

30

2 theda (degree)

Intensity (a.u.)

(c)

Kesterite CZTS

(338)

(d)

Raman Intensity (a.u.)

(a)

Intensity (a.u.)

20

(288)

Reference

(351) (354) (372)

Reference CZTS-170

200

Cu2-xS

250

300

350

400

Raman Shift (cm-1)

2 9 2 (h) 10 (eV/cm)

1.4 1.2 1.0 0.8

30

35

40

45

2 theda (degree)

50

55

60

43

44

45

46

47

48

49

(f) Reference CZTS-170

Reference CZTS-170

1.0

1.2

0.6

1.4

1.6

1.8

2.0

0.2

Eg1.40 eV

Eg1.35 eV

1.2

(338)

(e)

CZTS-170

(354)

(288)

(372)

250

300

350

Raman Shift (cm-1)

0.4

1.0

51

200

Energy (eV)

10

0.0

50

2 theda (degree)

Raman Intensity (a.u.)

25

PL Intensity (a.u.)

20

1.4

Band gap energy (h)

1.6

1.8

400